Compositional imaging by electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has been applied to map the distributions of calcium in cryosections of depolarized frog sympathetic neurons. These cells were found to accumulate Ca within their abundant mitochondria rather than in their endoplasmic reticulum in the period of a few minutes after depolarization. Elemental maps obtained by the EELS spectrum- imaging technique revealed accumulation of Ca in ~10 nm sized, phosphorus-rich inclusions. Quantitative analysis indicated that the Ca concentration in the inclusions was ~600 mmol/kg dry weight, while the concentration integrated over the whole mitochondria was ~30 mmol/kg. This latter value is in good agreement with parallel measurements by x-ray microanalysis. The EELS results suggest that, in sympathetic neurons, calcium accumulates in small intramitochondrial deposits under physiological conditions. This is a continuation of Intramural Research Project Z01-RR-10331-08 BEI.